In a particular type of high-density optical scanning device, a solid immersion lens (SIL) is used to focus a radiation beam to a scanning spot on an information layer of a record carrier. A certain size of an air gap between the exit face of the SIL and the entrance face of the record carrier, for example 25 nm, is desirable to allow evanescent coupling of the radiation beam from the SIL to the record carrier. Evanescent coupling may otherwise be referred to as frustrated total internal reflection (FTIR). Recording systems using evanescent coupling are also known as near-field systems, deriving their name from the field formed by the evanescent wave at an exit face of the SIL, which is sometimes referred to as the near field. An exemplary optical scanning device may use a radiation source which is a blue laser emitting a radiation beam having a wavelength of approximately 405 nm.
During scanning of the record carrier the evanescent coupling between the exit face of the SIL and the outer face of the record carrier should be maintained. This involves maintaining the size of the gap at a desired, very small value during motion between the SIL and the record carrier. An efficiency of this evanescent coupling in general varies with a change in the size of the gap between the exit face and the entrance face. When the gap size becomes larger than a desired gap size the coupling efficiency tends to decrease and a quality of the scanning spot will also decrease. If the scanning procedure involves reading data from the record carrier, for example, this decrease in efficiency will result in a decrease in the quality of the data being read, possibly with the introduction of errors into the data signal. Too small a gap size may result in collision of the SIL and the record carrier.
To allow control of the width of the air gap using a mechanical actuator at such small distances, a suitable control signal is required as input for the gap servo system. The gap signal is a signal that is a measure for the width of the gap between the exit surface of the objective system and the entrance surface of the optical record carrier. As disclosed in the paper by T. Ishimoto et al. [1] and Zijp et al. [2], a signal that is suitable as gap signal can be obtained from the reflected light with a polarization state perpendicular to that of the forward radiation beam that is focused on the record carrier. A significant fraction of the light becomes elliptically polarized after reflection at the SIL-air-record carrier interfaces: this effect creates the well-known Maltese cross when the reflected light is observed through crossed polarizers. Integrating all the light of this Maltese cross using polarizing optics and a radiation detector, which can be a single photodetector, generates the gap signal. The value of gap signal is zero for zero gap width and increases with increasing gap width and levels of at a maximum value when the gap width is approximately a tenth of the wavelength. The desired gap width corresponds to a certain value of the gap signal, the set-point. The gap signal and a fixed voltage equal to the set-point are input in a comparator, e.g. a subtractor, which forms a gap error signal at its output. The gap error signal is used to control the gap servo system.
Due to manufacturing tolerances of lens elements and assembly (such as thickness, mutual distance and radii of the optical elements) of the objective lens, it is very difficult to make a near field lens with its focus point at the desired position. As the gap width is preferably in the range of about 25 nm the requirements on the focus position of the system are in a similar range. The depth of focus is about λ/2NAeff2 (were the focused spot is just diffraction-limited), which results for a wavelength of 405 nm and a NAeff of 1.8 in about 63 nm.
The tolerance on the Lens-SIL distance for less than 15 mλ rms (milli-waves root-mean-square optical path difference (RMS OPD)) wavefront aberrations is only 0.25 μm (see paper by F. Zijp et al. [2]), which is extremely difficult to achieve in practice. Deviations of the vergence of the radiation beam from the design values for the beam vergence of the objective lens in use, may also affect the resulting position of the focus in the system. Besides this defocus also other aberrations, such as spherical aberration, can affect the position of the focus of the system.
All such errors may result in an erroneous gap signal. As the distance from the exit surface of the SIL to the record carrier is typically smaller than 1/10th of the wavelength of the radiation, there will be a risk of damaging the optical record carrier with the objective lens when the gap signal is not correct.
A current practice for the focus initialization of the optical recording system is based on a very tight tolerance on the vergence of the radiation beam, preferably a parallel radiation beam, towards the objective lens. Then the objective lens is mounted in the optical path. The objective lens is brought into contact with a non-rotating record carrier (e.g. a ROM disc) while focus and tracking servo control are not active (open loop). As the non-rotating record carrier is usually making small excursions, e.g. due to vibrations of the setup, modulations will be present in the signals of the tracking or RF-detection channel of the system. The focus offset of the system is then adjusted by means of vergence change of the radiation beam towards the objective lens (e.g. by means of telescope or collimator position adjustment) such that the gap signal is substantially zero when a readout signal (such as for example push-pull or data) with sufficient modulation are obtained. Now a gap signal corrected for focus offset is available and a startup of the system with a rotating disc using gap control and tracking control can be done (see [2]).
This offset adjustment can be obtained by changing the incoming laser beam from parallel to slightly convergent or divergent, for example by adjusting the collimator lens position or the position of a lens in a telescope configuration. For example, defocus aberrations due to a Lens-SIL distance error up to 20 μm were possible to be compensated in that way in the available optical recording system.
Larger errors may also be possible to correct in the same way; however, the resulting aberration level (mainly due to spherical aberrations) in the focused radiation beam will increase.
This means that it may be needed to optimize the collimator or telescope lens position for each manufactured objective lens. A possible alternative is to keep the collimator or telescope positions fixed such that the radiation beam towards the objective lens is a highly defined parallel radiation beam, and the Lens-SIL distance is adjusted by an additional actuator to minimize the defocus. This approach may, however, increase the complexity, cost and the moving mass of the lens system, which will reduce the bandwidth and therefore the achievable data rate. Especially when the near field optical recording system is to be commercialized and low-cost, compact and mass-producible optical pickup units (OPUs) are to be applied.
Another possible involves an interferometric measurement in the OPU. This is however a time-consuming measurement and is expected also to be difficult to perform because a lens with a NA larger than unity needs to be analyzed in a reflection set-up inside a compact build OPU.
Methods for initial focus optimization based on, for example, maximizing the central aperture or RF (data) signal during read-out of the record carrier may not be usable as an initial step as this already requires an initial focus setting as well as disc angle adjustments. A non-optimal focus position may lead to a highly aberrated spot, such that the gap control and/or tracking may easily fail. This can even leading to a crash between the objective lens with the record carrier when reading out the record carrier, which may lead to a damaged or unusable record carrier or objective lens.
The present focus initialization methods are therefore considered to be not robust and time consuming.
The main object of the invention is to provide a method an implementation for a robust initialization of the focus position in a near field or evanescent coupling optical recording system such that a reliable gap signal can be obtained.